Training sequence for linearizing an rf amplifier

ABSTRACT

There is disclosed a linearization training sequence that is transmitted by means of a radio frequency transmitter which is comprised in a mobile terminal or a base station of a radiocommunication system and transmits bursts according to a given frame structure. The sequence comprises a given number N of symbols which are part of an alphabet of symbols, N being an integer. At least one given number N1 of symbols of the sequence that is transmitted first is part of a sub-alphabet of symbols, which is contained in the alphabet of symbols, N1 being an integer that is smaller than or equal to N. The sub-alphabet of symbols consists of individual or a combination of symbols that provide the burst within which the sequence is transmitted with a narrower spectrum than the alphabet of symbols as a whole.

The present invention relates to the linearization of radiofrequency(RF) power amplifiers. It finds applications, in particular, in the RFtransmitters of the mobile terminals of digital radiocommunicationsystems. It may also be applied in the RF transmitters of base stationsin particular during the first start-up of such a station.

In current digital radiocommunication systems, one seeks to sendinformation with a maximum throughput in a given RF frequency band whichis assigned to a transmission channel (hereinbelow radio channel). To dothis, the modulations that have been used for a few years comprise aphase or frequency modulation component and an amplitude modulationcomponent.

Moreover, radio channels coexist in a determined frequency band allottedto the system. Each radio channel is subdivided into logical channels bytime division. In each time interval (or time slot), a group of symbolscalled a burst or packet is transmitted.

It is necessary to take care that, at each instant, the power leveltransmitted in each radio channel does not jam the communications in anadjacent radio channel. Thus, specifications prescribe that the powerlevel of an RF signal transmitted in a determined radio channel be, inan adjacent radio channel, less for example by 60 dB (decibels), thanthe power level of the RF signal transmitted in said determined radiochannel.

It therefore turns out to be necessary that the spectrum of the signalto be transmitted, which results in particular from the type of themodulation employed and the binary throughput, not be deformed by the RFtransmitter. In particular, it is necessary that the RF transmitterexhibit a characteristic of output power as a function of input power,which is linear.

However, the radiofrequency power amplifier (hereinafter RF amplifier)present in the RF transmitter has a characteristic that is linear at lowoutput power but nonlinear as soon as the power exceeds a certainthreshold. It is also known that the efficiency of the RF amplifier isall the better when working in a zone close to saturation, that is tosay in the nonlinear regime. Thus, the need for linearity and the needfor high efficiency (to save on battery charge) compel the use oflinearization techniques to correct the nonlinearities of the RFamplifier. Two of the techniques most commonly employed are basebandadaptive predistortion and the baseband Cartesian loop.

In the Cartesian loop technique, the signal to be transmitted isgenerated in baseband in the I and Q format. Additionally, a couplerfollowed by a demodulator makes it possible to tap off a part of the RFsignal transmitted and to transpose it to baseband (downconversion), inthe I and Q format. This baseband signal is compared with the basebandsignal to be transmitted. An error signal resulting from this comparisondrives a modulator, which provides for the transposition to theradiofrequency domain (upconversion). The output signal from themodulator is amplified by an RF amplifier which delivers the RF signaltransmitted.

In the baseband adaptive predistortion technique, the signal to betransmitted is generated in baseband, in the I and Q format, andpredistorted via a predistortion device. Then, this signal is transposedto the RF domain by virtue of an RF modulator. Next, it is amplified inan RF amplifier. A coupler followed by an RF demodulator make itpossible to tap off a part of the RF signal transmitted and to transposeit to baseband, in the I, Q format. This baseband demodulated signal isdigitized and compared with the baseband signal to be transmitted. Anadaptation of the predistortion coefficients, carried out during a phaseof training of the predistortion device, allows the demodulated I and Qformat signal to be made to converge to the I and Q format signal to betransmitted.

In both techniques, a part of the signal transmitted is tapped off atthe output of the RF amplifier so as to compare it with the signal to betransmitted. As a result, linearity is not obtained immediately but onlyafter a certain time, required for the convergence of the linearizationdevice. The signal transmitted has, during a period corresponding to thephase of training of the linearization device, a spectrum that iswidened by the uncorrected nonlinearities. It may not comply with theconstraints on the spectrum of the signals transmitted. This remarkapplies admittedly more to adaptive predistortion than to the Cartesianloop, even if the latter requires, in order to ensure its stability,initial adjustments of phase and of amplitude levels akin to training.

Several procedures have been proposed in the prior art for remedyingthis problem.

The procedure disclosed in WO 94/10765 relies on the transmission by thetransmitters of the system of particular sequences, so-calledlinearization training sequences, during linearization training phases.More particularly, training sequences are transmitted in an isolatedmanner in time intervals forming a particular logical channel of theradio channels, which is dedicated solely to linearization. As all thetransmitters of the system transmit their respective training sequenceat the same time, the communications are not disturbed by interferencebetween the radio channels which may possibly occur at this moment. Itis therefore not necessary to prevent interference between the radiochannels of the system.

However, this procedure has several drawbacks. Firstly, it requiresprior synchronisation of all the transmitters so that the lattertransmit their respective linearization training sequence in the logicalchannel dedicated to linearization. Moreover, no sending of data canoccur in the time intervals of this logical channel. Furthermore, at thestart of each transmission or in the event of a change of radio channel,the transmitter is compelled to wait for the next time interval of thelogical channel dedicated to linearization, unless the system is madeconsiderably more complex. This is why the temporal spacing between twotime intervals of said logical channel cannot exceed a second, so as toguarantee a certain quality of service (QoS). This technique istherefore very prejudicial to the spectral efficiency of theradiocommunication system. Finally, since no particular precaution istaken to avoid out-of-band transmission during the linearizationtraining phases, this technique may generate interference in respect ofthe transmitters of the other radiocommunication systems, which do notconform therewith.

Another procedure, disclosed in U.S. Pat. No. 5,748,678, provides forthe use during the linearization training phases of a second modulatorhaving half the throughput of the modulator normally used for thetransmitting of useful data. This second modulator generates a signalwhich possesses the same depth of amplitude modulation but a spectralwidth that is divided by two with respect to the signals transmittedoutside of the linearization training phases. This makes it possible toavoid jamming the adjacent radio channels by the signals uncorrected forthe nonlinearities which are transmitted during the linearizationtraining phases.

However, this procedure is fairly complex to implement since it requiresa second modulator, as well as associated filters or the use of adaptivefilters. This second modulator serves only during the linearizationtraining phases, that is to say for a very small fraction of the time.Specifically, when the amplifier has been linearized initially, itsuffices to correct any drifting of its characteristics. Thelinearization devices of the type alluded to in the introduction maydeal therewith in the course of the transmitting of useful data (at thenormal throughput). The overhead related to this corresponding secondmodulator is therefore hardly justified.

In order to alleviate the drawbacks of the aforesaid prior art, a firstaspect of the invention relates to a method of training a device forlinearizing a radiofrequency amplifier which is included within aradiofrequency transmitter of a first equipment of a radiocommunicationsystem, which transmitter is adapted for transmitting bursts accordingto a determined frame structure, each burst comprising symbols belongingto a determined alphabet of symbols. The method comprises the steps of:

a) generating a linearization training sequence comprising a determinednumber N of symbols, where N is a determined integer;

b) transmitting the linearization training sequence by means of theradiofrequency transmitter in at least certain of the bursts transmittedby the latter;

c) comparing the linearization training sequence transmitted with thelinearization training sequence generated so as to train saidlinearization device.

Advantageously, at least a determined number N1 of symbols of thelinearization training sequence sent first, where N1 is a determinedinteger less than or equal to N, belong to a subalphabet of symbolsincluded within said alphabet of symbols, said subalphabet of symbolsconsisting of symbols which, in isolation or combination, give the bursta narrower spectrum than said alphabet of symbols as a whole.

By subalphabet is understood to mean a part of the alphabet considered.Stated otherwise, if the alphabet comprises a determined number M ofsymbols (M-ary alphabet), the subalphabet comprises only a determinednumber M1 of these symbols (M1-ary subalphabet) where M and M1 areintegers such that M1 is less than M. The M1 symbols of the subalphabetare chosen in such a way as to give the RF signal which is transmitted anarrower spectrum than that given by the M symbols of the alphabet as awhole.

Said first equipment may be a mobile terminal or a base station of theradiocommunication system.

A second aspect of the invention relates to a device for training adevice for linearizing a radiofrequency amplifier which is includedwithin a radiofrequency transmitter of a first equipment of aradiocommunication system, which transmitter is adapted for transmittingbursts according to a determined frame structure, each burst comprisingsymbols belonging to a determined alphabet of symbols. The devicecomprises:

a) means for generating a linearization training sequence comprising adetermined number N of symbols, where N is a determined integer;

b) means for transmitting the linearization training sequence by meansof the transmitter in at least certain of the bursts transmitted by thelatter;

c) means for comparing the linearization training sequence transmittedwith the linearization training sequence generated so as to train saidlinearization device.

Advantageously, at least a determined number N1 of symbols of thelinearization training sequence sent first, where N1 is a determinedinteger less than or equal to N, belong to a subalphabet of symbolsincluded within said alphabet of symbols, said subalphabet of symbolsconsisting of symbols which, in isolation or combination, give the bursta narrower spectrum than said alphabet of symbols as a whole.

A third aspect of the invention relates to a mobile terminal of aradiocommunication system, comprising a radiofrequency transmitterhaving a radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, and which further comprises a device fortraining the linearization device according to the second aspect.

A fourth aspect of the invention relates to a base station of aradiocommunication system comprising a radiofrequency transmitter havinga radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, and which further comprises a device fortraining the linearization device according to the third aspect.

A fifth aspect relates to a linearization training sequence intended tobe transmitted by means of a radiofrequency transmitter of a mobileterminal or of a base station of a radiocommunication system, whichtransmitter is adapted for transmitting bursts according to a determinedframe structure. The sequence comprises a determined number N ofsymbols, where N is a determined integer, these symbols belonging to adetermined alphabet of symbols.

Advantageously, at least a determined number N1 of symbols of thelinearization training sequence sent first, where N1 is a determinedinteger less than or equal to N, belong to a subalphabet of symbolsincluded within said alphabet of symbols, said subalphabet of symbolsconsisting of symbols which, in isolation or combination, give the burstin which the linearization training sequence is transmitted a narrowerspectrum than said alphabet of symbols as a whole.

The object of the invention is therefore achieved by using a particulartraining sequence which allows the RF signal transmitted to preserve,during the linearization training phase, a spectral width compatiblewith the sought-after performance without any particular constraint onthe instants at which this training is carried out or on the complexityof the transmitter. The binary throughput during the linearizationtraining phase may be the same as that outside of this phase.

Other characteristics and advantages of the invention will becomefurther apparent on reading the description which follows. The latter ispurely illustrative and should be read in conjunction with the appendeddrawings in which:

FIG. 1 is a schematic diagram of an exemplary mobile terminal accordingto the invention;

FIG. 2 is a table illustrating an exemplary data modulation based on aquaternary alphabet of symbols;

FIG. 3 and FIG. 4 are graphs illustrating the effect of the choice ofthe symbols of the training sequence on the spectrum of thecorresponding RF signal respectively at the input and at the output ofthe RF amplifier;

FIG. 5 is a diagram illustrating an exemplary linearization trainingsequence according to the invention;

FIG. 6 and FIG. 7 are diagrams illustrating exemplary bursts transmittedby the mobile terminal, able to incorporate a linearization trainingsequence according to the invention.

Represented diagrammatically in FIG. 1 are the means of an exemplarymobile terminal according to the invention. Such a mobile terminalbelongs for example to a radiocommunication system which additionallycomprises a fixed network having base stations.

The terminal comprises a transmit chain 100, a receive chain 200, acontrol unit 300, a permanent memory 400, as well as an automatic gaincontrol device 500 (AGC) associated with an RF receiver of the receivechain 200.

The transmit chain 100 comprises a useful-data source 10, for example aspeech coder delivering voice-coding data. The source 10 is coupled toan M-ary data modulator 20 which provides for the baseband modulation ofthe data to be transmitted according to a modulation with M distinctstates, where M is a determined integer. The binary data which itreceives from the source 10 are translated by the modulator 20 intosymbols belonging to an M-ary alphabet, that is to say comprising Mdistinct signals. The output of modulator 20 is coupled to the input ofa radiofrequency transmitter 30. On the basis of the string of symbolsreceived, the transmitter 30 produces an RF signal suitable for radiotransmission via an antenna or a cable. The output of the transmitter 30is coupled to a transmit/receive antenna 40 via a switch 41. Thus the RFsignal produced by the transmitter is transmitted on the radio channelassociated with the transmitter.

The receive chain 200 comprises a radiofrequency receiver 50 which iscoupled to the antenna 40 via the switch 41, so as to receive an RFsignal. The receiver 50 provides for the transposition from the RFdomain to the baseband (downconversion). For this purpose, it comprisesa variable gain amplifier 59 the function of which is to compensate forthe power variations (which may be fast) on the antenna 40 so that theremainder of the receive chain processes a signal having a substantiallyconstant power level, thereby ensuring good performance. The receivechain 200 also comprises an M-ary data demodulator 60, coupled to thereceiver 50. The data demodulator 60 provides in baseband for thedemodulation of the data of the signal received, that is to say theoperation inverse to that provided by the modulator 20. Finally, thereceive chain 200 comprises a data consumer device 70, such as a speechdecoder, which is coupled to the demodulator 60. This device receives asinput the binary data delivered by the demodulator 60.

The unit 300 is for example a microprocessor or a microcontroller whichprovides for the management of a mobile terminal. In particular, itcontrols the data modulator 20, the data demodulator 60, the transmitter30 and the switch 41. It also generates signaling data which aresupplied to the modulator 20 so as to be transmitted in appropriatesignaling logical channels. Conversely, the unit 300 receives from thedata demodulator 60 signaling data dispatched by the fixed network inappropriate signaling logical channels, in particular synchronizationinformation and operating commands.

The memory 400 is for example a ROM (“Read Only Memory”), EPROM(“Electrically Programmable ROM”) or Flash-EPROM memory, in which arestored data which are used for the operation of the mobile terminal.These data comprise in particular a linearization training sequence towhich we shall return later.

An exemplary embodiment of the transmitter 30 will now be described. Inthis example, the transmitter 30 comprises a radiofrequency poweramplifier 31, a radiofrequency modulator 32 which provides for thetransposition from baseband to the radiofrequency domain (upconversion),a linearization device 33, a training module 34 associated with thelinearization device.

The output of the power amplifier 31 delivers the RF signal to betransmitted. This is why this output is coupled to the antenna 40 viathe switch 41. The input of the power amplifier 31 receives aradiofrequency signal delivered by the output of the radiofrequencymodulator 32. The input of the latter is coupled to the output of thedata modulator 20 so as to receive the string of symbols forming thebaseband signal to be transmitted, through the linearization device 33.The latter comprises for example a predistortion device comprising apallet (“look-up table”) which translates each value of the signal to betransmitted into a predistorted value. As a variant or as a supplement,the device 33 can also comprise means of amplitude slaving of the outputsignal from the transmitter 30.

The training module 34 carries out the training of the linearizationdevice 33 as a function of an input signal which reflects the RF signaldelivered by the output of the power amplifier 31. For this purpose, themodule 34 receives a part of this RF signal, which part is tapped off atthe output of the power amplifier 31 by means of a coupler 36. Asneeded, the module 34 provides for the baseband return of the RF signalthus tapped off. Although being represented entirely inside thetransmitter 30, the module 34 can, at least in part, be implemented bymeans belonging to the control unit 300, in particular software means.

Finally, the automatic gain control device 500 makes it possible, underthe control of the control unit 300 to dynamically vary the gain of thevariable gain amplifier 59 of the RF receiver 50, as a function ofinformation which is received from the fixed network, according to amethod known per se. By virtue of this method, the fixed network basestation with which the terminal is communicating, transmits atdetermined instants a determined sequence, called the AGC sequence. Thissequence is known to and recognizable by the mobile terminal. It allowsthe latter to measure the power of the signal received from the basestation and to deduce therefrom a control for the gain of the amplifier59. This method is implemented in the mobile terminal by the device 500under the control of the unit 300.

According to a symmetric method, provision is made for the transmitter30 to transmit an AGC sequence, so as to allow dynamic control, by thebase station, of the gain of a variable gain amplifier included in an RFreceiver of the base station. This sequence is known to and recognizableby the base station. It allows the base station to measure the power ofthe signal received from the mobile terminal and to deduce therefrom acontrol for the gain of the variable gain amplifier of the RF receiverof the base station.

In an exemplary implementation of the invention, the data modulator 20applies a so-called F4FM modulation (standing for “Filtered 4-stateFrequency Modulation”), which is a proprietary modulation but isundergoing standardization at the TIA (Telecommunications IndustryAssociation). This is a 4-state modulation or quaternary modulation,that is to say an M-ary modulation where here M is equal to 4. When thethroughput of the modulator 20 is equal to 8 kilo-symbols/s, the sendingof 8 symbols lasts 1 ms (millisecond). Stated otherwise, the sending ofa symbol lasts 125 μs (microsecond).

The table of FIG. 2 gives the correspondence between binary data andsymbols, which is applied by the F4FM modulation. Each symbolcorresponds to two data bits. The alphabet of symbols is composed offour symbols denoted −3, −1, +1 and +3. This quaternary alphabet isdenoted {−3, −1, +1, +3}. When a signal to be transmitted is generatedfrom the symbols of this alphabet, the RF signal has a spectrum ofdetermined width. Among these symbols, the symbols denoted −1 and +1form a subalphabet which, when it alone is used for the generation ofthe signal to be transmitted, gives the corresponding RF signal aspectrum of reduced width with respect to said determined width. Thissubalphabet is denoted {−1, +1}. This is an M1-ary subalphabet, withM1=2, all of whose symbols belong to the complete alphabet {−3, −1, +1,+3}. According to a characteristic of the F4FM modulation, the symbolsof the subalphabet {−1, +1} are also those which induce the lowestamplitude modulation depth.

The radiofrequency modulator 32 provides for the transposition of thesignal to be transmitted onto a carrier frequency at around 400 MHz(megahertz) or around 800 MHz, in a radio channel of width equal forexample to 8 kHz (kilohertz). The various radio channels of the systemare spaced apart for example by 12.5 kHz. Each radio channel issubdivided into traffic logical channels or signaling logical channelsby time division. In each time interval, a burst is transmittedaccording to a determined frame structure which it is not necessary todetail here.

The manner of operation of the mobile terminal during a phase oftraining, by the device 34, of the linearization device 33 will now bedescribed. Although it will not be mentioned each time in what follows,it is of course understood that the terms “training phase” and the terms“training sequence” refer to the training of the linearization device 33performed by the training device 34 under the control of the unit 300.

The method of training the device 33 comprises a step consisting ingenerating a training sequence comprising a determined number N ofsymbols, where N is an integer. This step is carried out by the datamodulator 20 under the control of the control unit 300. For thispurpose, the unit 300 reads a corresponding sequence of 2×N bits in thememory 400.

Next, still under the control of the unit 300, the training sequence istransmitted by means of the transmitter 30 in at least certain of thebursts transmitted by the latter, according to the frame structure ofthe system.

The training device 34 thus obtains the training sequence transmittedand compares it with the training sequence generated, and performsactions accordingly such as adaptations of predistortion coefficients orthe like of the linearization device 33, according to a specifiedtraining algorithm. This algorithm may be adaptive. One speaks ofteaching to designate these operations.

Represented in the graph of FIG. 3 is the spectrum of a bursttransmitted in a determined radio channel, outside of the trainingphase, in three different cases. In the first case, corresponding tocurve 1, only the symbols of the subalphabet {−1, +1} are used. In thesecond case, corresponding to curve 2, a majority of the symbols usedbelong to the subalphabet {−1, +1}, the others belonging to the alphabet{−3, −1, +1, +3} excluding the subalphabet {−1, +1} (that is to say tothe subalphabet {−3, +3} formed of the symbols −3 and +3). Finally, inthe third case, corresponding to curve 3, the symbols are distributedsubstantially uniformly in the complete alphabet {−3, −1, +1, +3}. It isobserved that the spectrum is all the narrower the higher the number ofsymbols which belong to the subalphabet {−1, +1}. In each case, thespectrum is centred on the central frequency Fo of the radio channel.

In the graph of FIG. 4, the same curves correspond to measurementsperformed at the output of the nonlinearized power amplifier 31, that isto say for example at the start of the training phase. The aboveobservation is still valid. Furthermore, one also observes, on comparingthe two figures, that in each case the spectrum is wider in FIG. 4 thanin FIG. 3. This extends from the nonlinearities of the radiofrequencytransmitter 30, in particular the power amplifier 31. This widening ofthe spectrum may imply the jamming of the adjacent radio channels,during the training phase.

As a result, in order to comply with the spectral constraints during thetraining phase, a first part at least of the training sequence isadvantageously generated from the subalphabet {−1, +1}. In this way, thecorresponding RF signal exhibits a spectrum of minimum width. When thesystem is properly dimensioned, this makes it possible not to disturbthe adjacent radio channels during the training phase and in particularduring the initial time span where the training algorithm has not yetconverged.

The sequence which gives such a spectrum is obtained by simulation or bymeasurement of the entire transmit chain. It may be, as in the exampleconsidered here, that this sequence implies that the amplitudemodulation depth is also reduced. It may even be that this reduction hasadverse effects on the results of the linearization algorithm and thatthe sequence chosen is thus not suitable. This is why it may benecessary to add a constraint on the amplitude modulation depth asregards the choice of the training sequence, so as to obtain acompromise between the spectral widening due to the nonlinearities ofthe power amplifier (to be minimized) and the amplitude modulation depthinduced by this sequence (to be maximized). These constraints arevariable as a function of the power amplifier used in the transmitchain. A possible procedure whereby this sequence can be chosen is tocommit to a digital optimization on the choice of N symbols of thesequence. The transmit chain is taken with all its defects withoutparticular linearization. This sequence generally being short (of theorder of about ten symbols), the optimization may be an exhaustivesearch for the N symbols making it possible to comply with theconstraints desired both on the spectral width and on the amplitudemodulation depth.

It is also possible to envisage alterations in the value of theamplitude modulation depth in the course of the training phase (betweenthe start and the end of the training sequence), in the case where thetraining algorithm is adaptive. Specifically, the disturbancesengendered by the spectral widening of the signal decrease alongside theconvergence of the algorithm, and it then becomes possible to relax thespectral constraint slightly so as to increase the amplitude modulationdepth of the RF signal transmitted.

In an example, if one chooses the N1 symbols transmitted first byselecting them within the subalphabet {−1, +1}, it is possible to choosethe N2 symbols transmitted last in such a way that at least certain ofthem belong to the alphabet of symbols {−3, −1, +1, +3} excluding saidsubalphabet of symbols {−1, +1}, that is to say to the complementarysubalphabet {−3, +3}, where N1 and N2 are integers less than N such thatN1 and N2 are less than or equal to N. During the transmission of theseN2 other symbols, that is to say after having operated the algorithm fortraining the linearization device 33 on the N1 symbols transmittedfirst, the transmit chain is already linearized approximately. Thelinearization is admittedly not total but then it makes it possible touse other symbols generating an RF signal of larger amplitude excursionwhile complying with the spectral width constraints.

Preferably, matters may be contrived such that a majority or even thetotality of these N2 other symbols belongs to the subalphabet {−3, +3}which, according to a property of F4FM modulation, produce a moresignificant amplitude modulation depth. In the case of anothermodulation, it may be preferable to tend to a substantially uniformdistribution of the symbols in the complete alphabet.

In an example, N1+N2=N. Of course, N1+N2 may be less than N, therebymaking it possible to envisage other symbols transmitted between said N1symbols transmitted first and said N2 symbols transmitted last, byproducing intermediate effects in terms of spectral width and amplitudemodulation depth.

It may be noted that for any modulation it is possible to find a signalsequence of fixed length N whose characteristics satisfy imposedconstraints in terms of spectral width, amplitude modulation depth,and/or others.

It may also be noted that the convergence of the known algorithms fortraining linearization devices is fairly fast. It follows that anexhaustive search for the optimal sequence by computer-based simulationmay be performed without any problem.

The diagram of FIG. 5 illustrates an exemplary training sequenceaccording to the principles presented hereinabove. In this example, thecomplete alphabet of symbols is the quaternary alphabet {−3, −1, +1, +3}of the F4FM modulation. Stated otherwise M is equal to 4. Additionally,M1 is equal to 2, the subalphabet giving the RF signal a reducedspectrum being {−1, +1}, N is equal to 10, N1 is equal to 6, and N2 isequal to 4. The N1 symbols transmitted first are for example the symbols+1, −1, +1, −1, +1, and −1, successively in this order. The signaltransmitted then has a spectrum of minimum width, the amplitudemodulation depth remains limited since not all the symbols of thequaternary alphabet are used. In order to take account of the actualamplitude modulation depth of a sequence of useful data, for theconvergence of the training algorithm, it suffices to slightly widen thespectrum over the end of the training sequence and to choose for examplethe N2 symbols transmitted last in the complete alphabet. The N2 symbolstransmitted last are for example the symbols −3, +1, +3, and −3,successively and in this order. In this example, the complete sequenceis therefore formed of the symbols +1, −1, +1, −1, +1, −1, −3, +1, +3,and −3 successively and in this order.

Training phases may be performed periodically or in some other fashion.Other constraints may have to be taken into account after the initialtraining phase, when it is entirely suitable to correct drifting of thetransmitter. The training sequence may therefore alter both in contentand in length. The number N is therefore not necessarily fixed from onetransmission of the training sequence to another. If an increase in thesize of the sequence poses problems (for example if the frame structureis fairly inflexible), then the size N of the sequence can be fixed andjust its content can be modified as a function of the alterations in theconstraints on the system.

The diagram of FIG. 6 illustrates an exemplary burst. In this example,the burst has a duration equal to 20 ms. It comprises firstly aramping-up 51 of 625 μs, comprising five padding symbols, to ensure thepower rise. The expression padding symbols is understood to mean thatthe binary data sent in this ramping-up are padding bits, that is to sayfor example a string of 0s. It next comprises a sequence ofsynchronization data 52 whose duration is equal to around 5 ms. Next, itcomprises a sequence of useful data 53. The useful data may bevoice-coding data or more generally traffic data, or signaling datadepending on whether the burst is transmitted on a traffic logicalchannel or on a signaling logical channel, respectively. It finallycomprises a ramping-down 54, again having five padding symbols for thepower drop. Optionally, a guard time is moreover envisaged after thetransmission of a burst, so as to guarantee the return to reception ofthe transmitter.

In one embodiment, the training sequence may replace the useful data ofthe bursts inside which it is transmitted.

So as not to make the frame structure overly complex, and in particularto avoid having to reserve a specific time interval for the training ofthe linearization device 33, the space occupied by the linearizationsequence may take up only part of the useful data of a burst. Thischaracteristic makes it possible to be able to transmit useful datarapidly in the reminder of the burst without having to wait for thefollowing time interval.

Other embodiments are conceivable. Specifically, in any frame structureprovision is made to transmit isolated bursts, in particular at eachchange of logical channel (occurring in particular at each turn around,that is to say switchover from a receive phase to a transmit phase ofthe terminal), with each change of RF frequency (when a frequency jumpfunctionality is implemented by the system), with each change oftransmission power rating, or else in other particular cases that wouldtake too long to detail here.

FIG. 7 shows an example of an isolated frame such as this comprising,before the synchronization sequence 52, an AGC sequence referenced 55which is transmitted by a first item of equipment (mobile terminal orbase station) so as to allow the dynamic control, by a second item ofequipment respectively base station or mobile terminal with which thefirst item of equipment is communicating, of the transmission power ofits receiver (see above). In this example, the sequence 52 and thesequence 55 each last only 1 to 3 ms. The other parts of the burst areunchanged with respect to the burst of FIG. 6. The sequence of usefuldata 53 may sometimes be shorter than in the case of a normal burstaccording to FIG. 6.

In one particularly advantageous embodiment, part of these isolatedbursts is used to allow the device 34 for training the radiofrequencytransmitter 32 to execute an algorithm for training the linearizationdevice 33. In the example of FIG. 7, the linearization sequence is forexample included in the aforesaid AGC sequence.

It is thus possible to use the time required for the transmission of thetraining sequence for other ends such as for example the tuning of theAGC at reception, according to the method alluded to above in regard tothe diagram of FIG. 1. Advantageously, the value of the symbols of theAGC sequence is not subject to any constraint (the AGC sequence simplyhas to be known to the fixed network). There is therefore completefreedom in choosing the symbols of the sequence, or at least part of thesymbols of the sequence, in such a way that these symbols form asatisfactory training sequence.

According to another advantage, the recurrence of the AGC sequence isadapted to the needs of the training of the linearization device 33.Specifically, the AGC sequence as the training sequence are preferablytransmitted at the start of a frame, and then upon a change of logicalchannel, upon a change of RF frequency and/or upon a change of powerrating. This is why it is particularly advantageous to combine thesesequences (these sequences forming just one single sequence, or one ofthem being included in the other), and to transmit them preferably asindicated hereinabove.

According to another advantage, the AGC sequence is situated as near aspossible to the signal power ramping-up, for example, just after thisramping. In this way, the training of the linearization device may becarried out as quickly as possible and thus disturb transmission for theleast possible time.

In all the embodiments, it is preferable for the length of the trainingsequence to be such that it does not occupy too large a portion of theburst so as to keep a maximum of symbols for the broadcasting of usefulinformation. This duration obviously depends on the sought-afteraccuracy of the training algorithm but a compromise between accuracy andduration often turns out to be necessary in order to preserve a maximumof useful information in the burst. A reasonable compromise is achievedwhen it represents around 5% of the total duration of the burst. In thecase of a 20 ms burst transmitted at a binary rate of 8 ksymbols/s, theduration of a training sequence of N=10 symbols is thus equal to 1.25ms, i.e. 6.25% of the total duration of the frame.

1. A method of training a device for linearizing a radiofrequencyamplifier which is included within a radiofrequency transmitter of afirst equipment of a radiocommunication system, which transmitter isadapted for transmitting bursts according to a determined framestructure, each burst comprising symbols belonging to a determinedalphabet of symbols, the method comprising the steps consisting in: a)generating a linearization training sequence comprising a determinednumber N of symbols, where N is a determined integer; b) transmittingthe linearization training sequence by means of the transmitter in atleast certain of the bursts transmitted by the latter; c) comparing thelinearization training sequence transmitted with the linearizationtraining sequence generated so as to train said linearization device,wherein at least a determined number N1 of symbols of the linearizationtraining sequence sent first, where N1 is a determined integer less thanor equal to N, belong to a subalphabet of symbols included within saidalphabet of symbols, said subalphabet of symbols consisting of symbolswhich, in isolation or combination, give the burst a narrower spectrumthan said alphabet of symbols as a whole.
 2. The method of claim 1,wherein the linearization training sequence comprises a determinednumber N2 of other symbols transmitted last, at least certain of whichbelong to the alphabet of symbols excluding said subalphabet of symbols,or N2 is an integer less than N.
 3. The method of claim 2, wherein amajority or the totality of said N2 other symbols transmitted lastbelong to the alphabet of symbols excluding said subalphabet of symbols.4. The method of claim 2, wherein N1+N2=N.
 5. The method of claim 1,according to which the number N is fixed.
 6. The method of claim 1,according to which the linearization training sequence occupies only apart of the burst in which it is transmitted.
 7. The method of claim 6,wherein the linearization training sequence occupies around 5% of theduration of the burst in which it is transmitted.
 8. The method of claim1, wherein the linearization training sequence is transmitted at thestart of the frame.
 9. The method of claim 1, wherein the linearizationtraining sequence is further transmitted during a change of logicalchannel, a change of frequency and/or a change of power rating of thefirst equipment.
 10. The method of claim 1, wherein the trainingsequence is included within or includes a sequence of symbols that isdesigned moreover to allow the dynamic control of the gain of avariable-gain amplifier of a radiofrequency receiver of a second item ofequipment of the radiocommunication system with which said firstequipment communicates.
 11. A device for training a device forlinearizing a radiofrequency amplifier of a radiofrequency transmitterwhich is included within a first equipment of a radiocommunicationsystem, which transmitter is adapted for transmitting bursts accordingto a determined frame structure, each burst comprising symbols belongingto a determined alphabet of symbols, the device comprising: a) means forgenerating a linearization training sequence comprising a determinednumber N of symbols, where N is a determined integer; b) means fortransmitting the linearization training sequence by means of thetransmitter in at least certain of the bursts transmitted by thetransmitter; c) means for comparing the linearization training sequencetransmitted with the linearization training sequence generated so as totrain said linearization device, wherein at least a determined number N1of symbols of the linearization training sequence sent first, where N1is a determined integer less than or equal to N, belong to a subalphabetof symbols included within said alphabet of symbols, said subalphabet ofsymbols consisting of symbols which, in isolation or combination, givethe burst a narrower spectrum than said alphabet of symbols as a whole.12. The device of claim 11, wherein the linearization training sequencecomprises a determined number N2 of other symbols transmitted last, atleast certain of which belong to the alphabet of symbols excluding saidsubalphabet of symbols, or N2 is an integer less than N.
 13. The deviceof claim 12, wherein a majority or the totality of said N2 other symbolstransmitted last belong to the alphabet of symbols excluding saidsubalphabet of symbols.
 14. The device of claim 12, wherein N1+N2=N. 15.The device of claim 11, wherein the number N is fixed.
 16. The device ofclaim 11, wherein the linearization training sequence occupies only apart of the burst in which it is transmitted.
 17. The device of claim16, wherein the linearization training sequence occupies around 5% ofthe duration of the burst in which it is transmitted.
 18. The device ofclaim 11, wherein the means for transmitting are adapted fortransmitting the linearization training sequence at the start of theframe.
 19. The device of claim 11, wherein the means for transmittingare adapted for transmitting the linearization training sequence duringa change of logical channel, a change of frequency and/or a change ofpower rating of the first equipment.
 20. The device of claim 11, whereinthe training sequence is included within or includes a sequence ofsymbols that is designed moreover to allow the dynamic control of thegain of a variable-gain amplifier of a radiofrequency receiver of asecond wherein equipment of the radiocommunication system with whichsaid first item of equipment communicates.
 21. A mobile terminal of aradiocommunication system, comprising a radiofrequency transmitterhaving a radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, further comprising a device for training thelinearization device as claimed in claim
 11. 22. A base station of aradiocommunication system comprising a radiofrequency transmitter havinga radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, further comprising a device for training thelinearization device as claimed in claim
 11. 23. A linearizationtraining sequence intended to be transmitted by means of aradiofrequency transmitter of a mobile terminal or of a base station ofa radiocommunication system, which transmitter is adapted fortransmitting bursts according to a determined frame structure, thelinearization training sequence comprising a determined number N ofsymbols, where N is a determined integer, these symbols belonging to adetermined alphabet of symbols, wherein at least a determined number N1of symbols of the linearization training sequence sent first, where N1is a determined integer less than or equal to N, belong to a subalphabetof symbols included within said alphabet of symbols, said subalphabet ofsymbols consisting of symbols which, in isolation or combination, givethe burst in which the linearization training sequence is transmitted anarrower spectrum than said alphabet of symbols as a whole.
 24. Thesequence of claim 23, further comprising a determined number N2 of othersymbols transmitted last, at least certain of which belong to thealphabet of symbols excluding said subalphabet of symbols, or N2 is aninteger less than N.
 25. The sequence of claim 24, wherein a majority orthe totality of said N2 other symbols transmitted last belong to thealphabet of symbols excluding said subalphabet of symbols.
 26. Thesequence of claim 24, wherein N1+N2=N.
 27. The sequence of claim 23,wherein the number N is fixed.
 28. The sequence of claim 23, wherein thealphabet of symbols is the alphabet {−3, −1, +1, +3} of the symbols ofthe so-called F4FM modulation.
 29. The sequence of claim 28, wherein theN1 symbols sent first belong to the subalphabet {−1, +1}.
 30. Thesequence as claimed in claim 24, wherein the N2 symbols sent last belongin the majority or even as a totality to the subalphabet {−3, +3}. 31.The sequence as claimed in claim 28, wherein the N2 symbols sent lastbelong in the majority or even as a totality to the subalphabet {−3,+3}.